1. Field of the Invention
The present invention relates generally to two-dimensional CCD (charge-coupled-device) image sensors of the frame-interline transfer (FIT) mode for use in video cameras, and more specifically to a technique for reducing dark currents.
2. Description of the Related Art
Conventional two-dimensional charge-coupled devices of the FIT mode comprise a matrix array of photo diodes, a plurality of vertical registers and a single horizontal register. The vertical registers are divided into an imaging part and a memory part. The imaging part of the vertical registers is connected to the photodiodes and the memory part is connected to the horizontal register. During a vertical blanking period, electrical charges, or charge packets developed by the photodiodes are transferred to the imaging part of the vertical registers and then transferred downwards to the memory part. Specifically, in the memory part, each charge packet is stored in a potential well. During a subsequent horizontal blanking period, the charge packets are shifted downwards through the memory part to the horizontal register row by row. However, the surface potential of the well is not "pinned" to the storage level of the charge packet, dark currents are generated in the charge transfer channels of the memory part during horizontal scan periods, resulting in a video signal of low signal-to-noise ratio.
In addition, the conventional FIT-mode CCD image sensors can only be used for interlaced scan format. Since computer's video displays employ the non-interlaced scan format, there exists a need for two-dimensional CCD image sensors that can be used in a non-interlaced scan mode.